Re: [PATCH v2 5/8] scsi: ufs: core: Enable multi-level gear scaling

[Ziqi Chen <quic_ziqichen@quicinc.com>]

On 1/24/2025 2:02 AM, Bart Van Assche wrote:
> On 1/22/25 11:41 PM, Ziqi Chen wrote:
>> We use memcpy() here is due to memcpy() can be faster than direct 
>> assignment. We don't worry about safety because they are same struct 
>> "ufs_pa_layer_attr" so that we can ensure the accuracy of number of 
>> bytes and member type.
> 
> The memcpy() call we are discussing is not in the hot path so it doesn't
> have to be hyper-optimized. Making the compiler perform type checking is
> more important in this code path than micro-optimizing the code.
> 
> Additionally, please do not try to be smarter than the compiler. 
> Compilers are able to convert struct assignments into a memcpy() call if
> there are good reasons to assume that the memcpy() call will be faster.
> 
> Given the small size of struct ufs_pa_layer_attr (7 * 4 = 28 bytes),
> memberwise assignment probably is faster than a memcpy() call. The trunk
> version of gcc (ARM64) translates a memberwise assignment of struct 
> ufs_pa_layer_attr into the following four assembler instructions (x0 and
> x1 point to struct ufs_pa_layer_attr instances, q30 and q31 are 128 bit
> registers):
> 
>          ldr     q30, [x1]
>          ldr     q31, [x1, 12]
>          str     q30, [x0]
>          str     q31, [x0, 12]
> 
> Thanks,
> 
> Bart.
> 
Sure , Let me try and test it. If works fine , I will update in next 
version.

-Ziqi